1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a printer or a color copying machine.
2. Description of the Related Art
In recent years, an increase in image formation speed of electrophographic color image forming apparatuses has increased the types of tandem color image forming apparatuses. A tandem color image forming apparatus includes a photosensitive drum and developing devices, and successively transfers images of different colors onto a recording medium or an image conveying belt. The number of developing devices is the same as the number of coloring materials. The tandem color image forming apparatus is known to have a plurality of factors that cause misregistration. Accordingly, various methods are proposed to deal with these factors.
One factor involves ununiformity and mounting displacement of a lens in a deflection scanner and displacement of the deflection scanner when it is mounted to the body of the color image forming apparatus. In this case, a scanning line is inclined and bent. The inclination and bending depend upon color, thereby resulting in misregistration.
Japanese Patent Laid-Open No. 2002-116394 discusses a method of overcoming misregistration. In the method, in the step of assembling a deflection scanner, the bending amount of a scanning line is measured with an optical sensor, a lens is mechanically rotated to adjust the bending of the scanning line, and then the deflection scanner is secured to an image forming apparatus body with an adhesive.
Japanese Patent Laid-Open No. 2003-241131 discusses another method. In the method, in the step of mounting a deflection scanner to a color image forming apparatus body, the inclination of a scanning line is measured with an optical sensor, the deflection scanner is mechanically inclined to adjust the inclination of the scanning line, and then the deflection scanner is mounted to the color image forming apparatus body.
Japanese Patent Laid-Open No. 2004-170755 discusses still another method. In the method, the inclination and bending amount of a scanning line are measured with an optical sensor, and bitmap image data is corrected so as to cancel the inclination and the bending to form an image based on the corrected data. Since this method allows misregistration to be electrically corrected as a result of processing the image data, it does not require a mechanical adjuster or an adjusting step during the assembly. From these two points, this method allows misregistration to be corrected at a lower cost compared to the methods discussed in Japanese Patent Laid-Open No. 2003-241131 and 2003-241131. There are two methods of electrically correcting misregistration. One method is performed in one pixel unit and the other method is performed in less-than-one pixel unit. In the correction in one pixel unit, pixels are shifted in a subscanning direction in one pixel unit in accordance with the amounts by which the inclination and bending are corrected. In the correction in less-than-one pixel unit, gradation values of bit image data are adjusted for front and back pixels in the subscanning direction. By this correction, it is possible to eliminate an unnatural step at a shifted boundary resulting from the correction in one pixel unit, so that an image can be smoothed.
However, correcting misregistration by the method that is discussed in Japanese Patent Laid-Open No. 2004-170755 may cause a density variation in a fine image. The density variation of a fine image will be described with reference to FIG. 14. An input image 601 is a thin line of one dot. When an image 602 produced by performing color misregistration correction on the input image 601 is actually formed, an output image resulting from the correction of the color misregistration becomes a thin-line image having an ununiform density even though the input image 601 is a thin-line image having a constant density. This is caused by the electrophotographic image forming apparatus not being generally good at forming an isolated pixel with an image gradation value and an actual image density value remaining proportional to each other. Accordingly, this weakness causes noticeable density variation to occur in the fine image formed by a thin line.